A major source of the platinum group metals (platinum, iridium, osmium, palladium, rhodium, and ruthenium, hereinafter "PGM") is frequently associated with Cu-Ni deposits, and these metals usually occur in conjunction with nonferrous metal sulfide ores. Another source of PGM which is becoming important, especially in the U.S., is the secondary source, namely, scrap of various computer parts, printed circuit boards, connectors, and auto-exhaust catalysts.
PGM are traditionally recovered by aqua regia or HCl/Cl.sub.2. Almost all sulfidic PGM ores that are treated today are subject to a concentration procedure which includes froth flotation. Flotation concentrates are typically subjected to roasting in which the majority of sulfur is removed. Such an up-grading process is necessary because PGM associated with sulfide ores are frequently refractory. Another method is the direct smelting of the flotation concentrate to produce matte. This process is usually carried out at 1250.degree. C.-1350.degree. C. resulting in expensive operating and capital costs. The roasted and smelted products are then subjected to leaching. First, leaching of copper and nickel in a mild acidic environment is carried out with subsequent leaching in either aqua regia or HCl/Cl.sub.2 to extract platinum, palladium and other PGM. Because platinum-group metals are very inert, their extraction is very expensive. For example, the extraction of these metals from automobile catalysts is known to be notoriously expensive because of the high cost associated with reagent consumption. The methods used to process these metals tend to dissolve even silica and alumina, which are frequently the base matrix of platinum-group metals. As a result, the process suffers from high acid consumption and severe acid corrosion problems.
Almost all gold currently produced from ores in the world is extracted by cyanidation leaching. This method of extraction, however, has several drawbacks. First, cyanide is an extremely toxic compound, creating the need for expensive transportation, storage, and cleanup procedures. Furthermore, the leaching kinetics of gold and silver with cyanide is very slow. Leaching residence time is typically 2-4 days.
In addition, direct cyanidation leaching of refractory gold-bearing ores is not successful. One of the major challenges currently experienced by the extractive metallurgy industry is the efficient recovery of precious metals from refractory ores. There have been various ways practiced in industry to treat sulfidic refractory ores. Commonly practiced techniques for treating sulfide ores include high temperature roasting of the ore followed by leaching. Another technology utilizes pressure oxidation of these ores in an autoclave followed by extraction of precious metals from the ore. The cost of these two-stage processes is found to be excessive and, therefore, researchers have long been looking for alternative ways to treat this refractory ore. In the case of carbonaceous refractory ores, the carbonaceous material is destroyed or treated before extraction of gold is attempted. The carbonaceous material is frequently subjected to oxidation using ozone, chlorine, sodium hypochlorite, permanganates, perchlorates, and oxygen. Chlorine is currently used in the treatment of these carbonaceous ores. However, due to the high cost of chlorine, it is desirable to find an alternate way of treating this ore.
In the past, extraction of gold from gold bearing ores included the use of halogen elements. U.S. Pat. Nos. 3,957,505 and 4,557,759 deal with the hydrometallurgical recovery of gold from materials containing gold by introducing elemental iodine and iodide into the solution containing such materials.
U.S. Pat. Nos. 3,709,681, 3,826,750, and 3,778,252 deal with the extraction of precious metals such as platinum, palladium and gold from noble metal- containing materials by mixtures of organic solvents and a halogen element or compounds at ambient temperature. Although these processes are concerned with the extraction of precious metals from different sources using iodide and/or iodine, the medium used in these processes is organic and the conditions used are very different, and the environments in which these metals are associated with are very limited compared with those of the current invention.
U.S. Pat. No. 4,319,923 deals with the recovery gold and/or palladium from iodide-iodine etching or similar solutions. These technologies teach an art of separating precious metals from other metals in the solution, once they are in solution in the form of dissolved species.
Accordingly, a need has developed to extract platinum group metals, gold and silver from various materials in an efficient and economically advantageous manner. In response to this need, the present invention provides a method of extracting precious metals from ores or other precious metal containing materials such as scrap which overcomes the disadvantages noted above in the prior art. The inventive extractive method avoids problems with corrosion and acid soluble species by extracting the precious metals at increased pH levels. The inventive extractive method permits recovery of precious metals from previously difficult to work with refractory and carbonaceous gold-bearing ores.
The inventive extracted method is also very effective in extracting both platinum group metals and gold and silver under conditions of high temperature and pressure to improve precious metal recovery.